LAUSR

This paper investigates the design of a compact multiple-input multiple-output (MIMO) ground-radiation antenna system for 5G terminal devices. Ground-radiation antennas are small loop-type resonators that can excite the ground plane as radiators while having advantages of small size and convenient tunability, but their compact MIMO systems remain as unsolved issues. In this study, a novel and straightforward decoupling technique based on a closed-decoupling-loop is presented for MIMO applications in the first of its kind. The proposed closed-decoupling loop can be viewed as a coupling transformer between two antenna elements by acting as a half-wavelength loop-type resonator and by introducing a current null at its center; in this way, a tremendous isolation improvement is accomplished even though the distance between two antennas is only 1 mm. Therefore, a compact $2 \times 2$ MIMO ground-radiation antenna system and its $8 \times 8$ MIMO system are demonstrated for 5G applications. Both simulation and measurement are taken to validate the proposed decoupling technique and the compact MIMO ground-radiation antenna system.